I just purchased a Caltar II-N 1:5.6 f=210mm lens of eBay. It appears to be in great shape and the copal 1 shutter seems to be right on. The lens coating appears to be intact and the glass looks fine, however, there was a lot of dust and crud on the front and rear lens. After a careful cleaning I notice there remains a lot of dust on the rear glass of each element. Can anyone advise how best to take this lens apart for cleaning and what cleaning method is most effective while at the same time safe? Thanks for your help on this.